1. Field of the Invention
The present invention relates to a composition for a protective film, a color filter-provided substrate prepared by using it, and a liquid crystal display device employing the substrate.
2. Discussion of Background
By virtue of their characteristics such as thinness, lightness in weight and low consumption of electric power, light receptive display devices such as liquid display devices have found remarkable developments as display devices which can be substituted for CRTs (cathode ray tubes) which used to be main products of conventional display devices. It is a color filter that is essential to provide colors to such display devices and thereby to obtain a display performance closer to CRTs. Various materials and systems have been proposed for such a color filter.
A typical example is a dyed color filter obtainable by dyeing a thin film of a polymer such as gelatin with a dye, and such a dyed color filter has been used for a small size color television of liquid crystal system. This dyed color filter is excellent in transmittance and chromaticity. However, as the trend for large size liquid crystal display devices has progressed, some drawbacks have been pointed out, including e.g. inadequate uniformity of chromaticity over a large area, high costs resulting from the complexity of the process for its production and inadequate heat resistance or weather resistance resulting from the use of a dye.
To overcome such drawbacks, various methods for producing color filters have been proposed in recent years, including e.g. a so-called pigment dispersion method wherein a colorant is preliminarily dispersed in a photosensitive resin, and patterning is carried out by photolithography, a printing method wherein a pattern is directly formed on a substrate by screen, off-set or gravure printing using an inorganic or organic pigment, and an electrodeposition method.
However, such color filter-provided substrates have a problem that there will be nonuniformity in thickness (usually at least 0.1 .mu.m) in each color or among different colors. In particular, by the above-mentioned pigment dispersion method, there will be substantial nonuniformity in thickness due to overlapping among different colors, while it is thereby possible to form a color filter with a high precision in patterning. Such nonuniformity in thickness causes a problem such that when such a color filter-provided substrate is used as an electrode substrate for a liquid crystal display device, no adequate contrast or gradation is obtained.
For such a color filter, it is common to form a transparent protective layer on the colored layer for the purpose of e.g. protecting the colored layer. Such a protective layer simultaneously has a function for leveling the colored layer to some extent. Accordingly, it is important to use as a protecting layer the one having a high leveling function. Other properties which the protective layer is required to have, include adhesion to a transparent electrode (such as ITO), patterning efficiency by etching, proper hardness, adhesion to glass and adhesion to sealing materials used for liquid display devices.
For such a protective layer, several materials have been proposed including, for example, acrylic type, polyimide-polyamide type and silicone type materials.
Acrylic materials are disclosed, for example, in Japanese Unexamined Patent Publications No. 17124/1986, No. 141401/1986 and No. 89022/1987. However, they usually have a drawback that their leveling function is inadequate. Further, they have another drawback that their adhesion to transparent electrodes and patterning efficiency are inadequate. Namely, when a transparent conductive film is formed on the protective layer and fine processing (patterning) is applied by photolitho process-wet etching treatment, no adequate adhesion between the protective film and the transparent conductive film tends to be obtained, whereby there has been a problem such that peeling occurs, or undercutting or side etching is substantial during the patterning of the transparent conductive film.
Polyimide-polyamide type materials are disclosed, for example, in Japanese Unexamined Patent Publications No. 78401/1985 and No. 77007/1986. These materials are usually superior to acrylic materials in the above-mentioned patterning efficiency and adhesion to a transparent electrode, but they have a drawback that their leveling function is extremely poor, and their costs are high. Therefore, these materials are not so commonly used.
Further, with respect to silicone type materials, thermosetting materials are disclosed, for example, in Japanese Unexamined Patent Publication No. 218771/1988. However, their leveling function is inadequate. Besides, they have problems such that no adequate adhesion between the protective film and the transparent conductive film tends to be obtained, and peeling occurs and undercutting or side etching is substantial during the patterning of the transparent conductive film.
As another attempt to level or flatten the surface of the protective layer, it has been proposed to polish the surface of the protective layer, for example, in Japanese Unexamined Patent Publications No. 193781/1989 and No. 206304/1989.
However, this method requires a step of polishing which is poor in the productivity and thus causes an increase of the costs. Further, this method has another problem that scratching marks are likely to result on the protective layer by polishing.
Recently, it has been proposed to use a protective layer made of a composition having a polyfunctional acrylate incorporated to an epoxy acrylate (Japanese Unexamined Patent Publications No. 51202/1992 and No. 100003/1992). However, such a protective layer has a problem that the hardness is so high that cracking is likely to result. Further, it has also been proposed to incorporate from 13 to 20 wt % of an epoxy resin to the above composition (Japanese Unexamined Patent Publication No. 51203/1992). However, with such a protective layer, there is a problem that no adequate adhesion can be secured between the protective layer and the substrate or the sealing portion when a liquid crystal cell is assembled.